FIELD OF THE INVENTION
The invention relates to a system for contactless power and data transmission.
Such systems include a stationary station and a mobile part, for example in the form of a smart card or a chip key, etc. The stationary station supplies power which is necessary for operation of the mobile part. A loosely coupled coil pair, in which each coil is assigned to a resonant circuit in the mobile part and the stationary part, is generally provided for that purpose. A radio-frequency signal is used for the power transmission. In the mobile part, that signal is fed to a rectifier unit and, if appropriate, to a voltage regulating unit which is connected downstream of the rectifier unit. The DC voltage obtained therefrom is used to supply the mobile part with power. Such systems are employed, for example, in recognition systems, immobilizers, smart cards and recognition systems for identifying useful animals, etc.
In the case of such systems with contactless power transmission, the transmission efficiency is highest when the transmission frequency of the primary circuit corresponds to the resonant frequency of the secondary circuit. The primary circuit contains, for example, the resonant circuit of the stationary part, and the secondary circuit that of the mobile part. Due to parameter fluctuations of the components and interference influences coupled in externally, that requirement can only be achieved within the sum of component parameters and other influencing quantities. Tuning of the secondary circuit is necessary in order to achieve the best possible signal and power transmission.
German Patent DE 38 10 702 C2 specifies a device for contactless signal and power transmission in which the stationary station contains a tunable resonant circuit that is driven by a control circuit in order to carry out tuning to the receiving circuit of the mobile part, thereby achieving the best possible reception. The stationary station contains a further resonant circuit. The signals which can be picked off from the resonant circuits are evaluated with regard to the relative phase angle with respect to one another. Series-connected variable-capacitance diodes which, for their part, are connected in parallel with the resonant circuit of the mobile station that is to be tuned, are controlled in dependence on that evaluation.
Published International Patent Application WO 93/23908 describes a system for contactless power and data transmission between a primary station and at least one mobile secondary station, in which system the secondary resonant circuit in the mobile station is tuned to the primary resonant circuit in such a way that capacitors can be connected in parallel. In order to set the tuning, it is proposed to derive a voltage signal from the reception frequency. The voltage signal is compared with a reference signal. A switching signal for the optional switching-in of the capacitors is derived from the result of the comparison.